The so-called "Yagi-Uda antenna" has been successfully used for many years in applications such as reception of television signals, point-to-point communications, and certain types of military electronics. The Yagi-Uda antenna can be designed to have high directivity or gain and low voltage-standing-wave ratio ("VSWR") throughout a narrow band of contiguous frequencies. It is also possible to operate the Yagi-Uda antenna in more than one band of frequencies provided that each band is relatively narrow and provided further that the mean frequency of one band is an odd multiple of the mean frequency of another band.
In the Yagi-Uda antenna, there is a single element which is driven from the source of electromagnetic energy. That element is commonly a half-wave dipole. Arrayed with the dipole element are certain parasitic elements, typically a so-called "reflector" element on one side of the dipole, and a plurality of so-called "director" elements on the other side of the dipole. The director elements are usually disposed in spaced relationship in the portion of the antenna pointing in the direction to which electromagnetic energy is to be transmitted, or from which signal energy is to be received in the case of a receiving antenna. The reflector element, on the other hand, is disposed on the side of the dipole opposite from the array of director elements.
During the period of time since the introduction of commercial television, a great deal of effort has been exerted to design Yagi-Uda antennas having optimum directivity at a single frequency or near-optimum directivity over some specified bandwidth of frequencies. The approach to such optimization was explained in a paper by Dr. David K. Cheng, published in the Proceedings of the Institute of Electrical and Electronics Engineers, Volume 59 No. 12, December 1971, entitled "Optimization Techniques for Antenna Arrays." Further material directed to the optimization of Yagi-Uda antennas was published by Dr. Cheng together with C. A. Chen in the Transactions of the Institute of Electrical and Electronics Engineers on Antennas and Propagation, Volume AP-21, No. 5, September 1973 and Volume AP-23, No. 1, January 1975. One of the papers by Cheng and Chen related to the optimization of the spacing of the parasitic elements in Yagi-Uda antennas. The other paper related to optimization of the lengths of the parasitic elements in such antennas. By using so-called "perturbation techniques", Cheng and Chen were able to adjust the inter-element spacings and the lengths of the elements to obtain relatively high directivity over a narrow band of frequencies. In this way, Cheng and Chen achieved a directivity of 9.9 dB.+-.2.1 dB over a twenty-nine-percent bandwidth, but the voltage-standing-wave ratio achieved by Cheng and Chen in this way maintained a value less than 3.0 to 1 over only a nineteen-percent bandwidth.
In U.S. Pat. No. 2,688,083, Elmer G. Hills disclosed a way of configuring a Yagi-Uda antenna to achieve coverage of two relatively narrow frequency bands which were non-contiguous with each other. In 1950, when Mr. Hills filed the application on which the aforementioned patent was granted, there were only two frequency bands authorized for commercial television in the United States. The lower frequency band extended from 54 megahertz to 88 megahertz, while the higher frequency band covered the range between 174 megahertz and 216 megahertz. Taking advantage of the fact that the mean frequencies in those respective bands were related to each other roughly in the ratio of 1 to 3, Mr. Hills ingeniously devised a way to cover both bands with a single antenna. However, the frequencies between the two bands were almost entirely outside the receiving capability of the antenna disclosed and claimed in his patent. He did not achieve high directivity over a relatively wide band of contiguous frequencies.